1. Field of the Invention
This invention generally relates to electronic packaging and assembly. More particularly, this invention relates to electronic interconnect technology, specifically to solder interconnects on copper/electroless nickel/immersion gold metallization surfaces.
2. Description of the Related Art
Electrically conductive interconnects have been used for years within the semiconductor industry in all manner of attachment applications. They are used, for example, to make both first level (integrated circuit (IC) to package) connections and second level (electronic component package to substrate) connections. Examples of simple conductive interconnects include: pin-through-hole, wirebonding, tape automated bonding (TAB), leaded surface mount, bump grid array, and flip-chip interconnects.
As semiconductor technology and printed circuit board (PCB) technology have advanced and semiconductor devices have become more complex and powerful, the need for smaller electrically conductive interconnects for use in high density applications has likewise grown. As interconnects have become smaller and more densely packed, it has become necessary to have flatter interconnect attachment surfaces in order to ensure reliability.
The common method for forming solder pads on substrates is hot air solder leveling (HASL). However HASL produced solder pad surfaces that are not flat enough to be used reliably in all high-density applications. Several alternatives to HASL are now being used in the PCB and semiconductor industries, including organically coated copper (OCC) and copper/electroless nickel/immersion gold metallization (CENIGM). OCC creates sufficiently flat solder pads, but is difficult to test by probing and does not hold up well during multiple soldering reflow cycles.
The reliability of Tin-Lead (Sn--Pb) solder joints formed on CENIGM solder pads has been tested by semiconductor and printed circuit assembly (PCA) industries. The testing indicated Sn--Pb solder joints formed on CENIGM solder pads are subject to interfacial brittle fracture failure during mechanical loading. Such mechanical loading might include, for example, bending or vibrating a printed circuit board (PCB) with an IC package such as a bump grid array (BGA) package attached, dropping a PCB on the floor, or prying a BGA off with a screw driver. The solder joints formed on CENIGM solder pads were found to fracture under bending at loads significantly lower than solder joints formed on OCC or HASL pads (Z. Mei, M. Kaufmann, A. Eslambolchi, and P. Johnson, "Brittle interfacial fracture of PBGA packages soldered on electroless nickel immersion gold", 48th ECTC conf. proceeding, 1998, pp. 952-961; E. Bradley and K. Baneji, "Effect of PCB finish on the reliability and wettability of BGA packages", 45th ECTC conf. proceeding, 1995, pp. 1028-1030; and E. Bradley, lecture in Ninth Annual Soldering Symposium, SUNY-Binghamton, Oct. 21, 1996).
The fracture of solder joints on CENIGM occurred by a cleavage within the bonding intermetallic layer between Sn--Pb solder and CENIGM, while the strength of solder joints on OCC and HASL was much stronger and fracture of these solder joints occurred by peeling the solder pads off PCB. The root cause of the brittle interfacial fracture of solder joint on CENIGM is not been known, although the phosphorus in electroless nickel is suspected, since the brittle interfacial fracture does not occur in solder joints on Cu/electrolytic Nickel/Electrolytic Gold surface finish.
Because the phenomenon of brittle interfacial fracture of solder joints on CENIGM is being extensively observed in PCA industry, an extensive effort to find the root cause and solution for the problem of brittle interfacial fracture by PCB fabricators, plating chemistry and equipment suppliers, and original equipment manufacturers (OEMs).
Additionally, poor wetting and adhesion strength of both Pb--Sn and Lead-Indium (Pb--In) solder bumps on the CENIGM in solder bump flip-chip interconnections has been observed (K. Puttlitz, "Preparation, structure, and fracture modes of Pb--Sn and Pb--In terminated flip-chip attached to gold capped microsockets", IEEE Trans-CHMT, vol. 13, 1990, p. 647-655). The poor wetting, is thought due to a precipitate phase, Ni.sub.3 P and was also described by V. F. Hribar, J. L. Bauer, and T. P. O'Donnell, "Microstructure of electroless nickel-solder interactions," 3rd International SAMPE Electronics Conf., Jun. 20-22, 1989, pp. 1187-1199.
It is not clear if the brittle interfacial fracture problem and the poor wetting problem have the same root cause. Thus far, no solution to the brittle interfacial fracture problem has been proposed, while several solutions to the problem of weak solder joint formation on CENIGM solder pads due to poor wetting have been proposed. One solution for the poor wetting and adhesion strength of both Pb--Sn and Pb--In solders on CENIGM is coating the electroless Nickel (Ni) with a layer of gold (Au) 3 to 4 times thicker than the standard immersion gold thickness. The Ni and Au are then heated to 600-650.degree. C. to convert the Au layer into a Au--Ni alloy which is not susceptible to molten solder consumption as is pure Au. This solution is not readily adaptable applicable to CENIGM solder pads formed on standard PCB materials since most PCBs cannot survive at the temperatures required to form the Au--Ni alloy.
A second solution to the problem can be found in U.S. Pat. No. 4,603,805 entitled, "Method for enhancing the solderability of nickel layers." The patent describes a method of enhancing the solderability of phosphorus containing Ni by heating the Ni surface to a temperature above 347.degree. C. first in an oxidizing atmosphere and then in a reducing atmosphere prior to soldering. The oxidation treatment converts the phosphorus (P) to phosphorus oxides (P.sub.2 O.sub.3, P.sub.2 O.sub.3, or P.sub.2 O.sub.5). These oxides sublime or boil away from the surface. This solution requires expensive equipment to implement, extensive time for processing, and requires temperatures higher than most PCBs can tolerate.
In addition, saccharin has been reported to be able to greatly enhance the solderability of electroless Ni deposit (J. L. Fang, X. R. Ye and J. Fang, Plat. and Surf. Fin., July 1992, p.44).
Finally, there has been some indication that increased wetting of Sn--Pb solders on electroless Ni can be accomplished by aging the solder joint at 150.degree. C. for 30 minutes, or 250.degree. C. for 30 minutes for more significant improvement (K. L. Ling and J. M. Jang, "Wetting behavior between solder and electroless nickel deposits," Materials Chemistry and Physics, vol. 38, 1994, pp. 33). This solution subjects sensitive electronic components to high temperatures for relevantly long periods of time and extends the component attachment process from one that normally takes seconds to one that takes half an hour.
Accordingly, it is apparent that there is a need for a high-strength solder interconnect on CENIGM that can be formed using current solder interconnect processes, equipment, and temperatures, and which doesn't add significant time or expense to the manufacturing process.